@rosa Hi welcome to the forum. Well at the moment Pencil2D is implementing copy / paste functionality for frames but it won’t be added until the next version.
So there are a few things you can you:
Manually duplicate each frame of the loop until you fill the desired length. F6 is the shortcut to duplicate a frame and to duplicate a frame you need to place the vertical playhead on the first frame you want to duplicate, when you press the shortcut it will spawn a new copy in the next available empty space.
Con: You have to do it manually and sequentially.
Export the specific frames as a PNG sequence with transparent background. Then reimport these frames into the same layer.
Con: It’s annoying to use the import dialogue every time.
import PCLX function in File > Import > Layers from project file
This will allow you to bring a layer with all its frames into Pencil2D and you can import it as many times as you want.
Con: all the frames for the “loop” will be in different layers, and since this is a new feature it might crash if you’re not using version 0.6.6 or a nightly build (development versions)
For now I might recommend option #2 since it’s easy to export the frames, then copy and paste the files themselves in the file browser and then once you renumber them you can reimport the entire sequence quickly.